It's such a shame that we didn't see her brother... ugh the older sister is hot but bb g n...

I know I could be totally off-base, but I think the older sister set a trap to kill their parents, too...

I feel like it was not the sis who killed them all

If not, she shouldn't have kept that information to herself and force the MC to hate her for no reason. If she was really concerned about being overheard, then she should have brought the MC to a room and even just whispered about the situation in her ear.